Nasal CPAP Treatment of OSA
Nasal Continuous Positive Airway Pressure (CPAP) treatment of Obstructive Sleep Apnea (OSA) was invented by Sullivan. See U.S. Pat. No. 4,944,310. Apparatus for treating OSA typically comprises a blower that provides a supply of air or breathable gas to a patient interface, such as a mask, via an air delivery conduit. Since patients typically sleep while wearing the device, it is desirable to have a system which is quiet and comfortable.
Generic Blower/Air Pump
Blowers are typically classified as centrifugal, axial or mixed flow. Generally, blowers comprise two main parts: a rotating part, namely an impeller and shaft; and a stationary part that defines a fluid flow path, typically a chamber such as a volute. Rotation of the impeller imparts kinetic energy to the air. The stationary part redirects the air expelled from the impeller into an enclosed outlet passage. During this redirection, resistance is encountered to flow because of the pressure generated by downstream resistance or a downstream pressure source. As the flow is slowed against this resistance, a portion of the kinetic energy is converted to potential energy in the form of pressure.
Generally, the faster the impeller is rotated, the higher the pressure that will be developed. A less effective blower must rotate its impeller faster to generate the same pressure as a more effective blower. Generally, running a given blower slower makes it quieter and prolongs its life. Hence, it is generally desirable to make blowers more effective at generating a supply of air at positive pressure.
With reference to FIGS. 1 and 2, three directions are defined, i.e., radial R, tangential T and axial A. Prior art centrifugal blower 10 includes an outlet 20, an inlet 30, an electric motor 40, an impeller 50 and a shaft 60. Arrows 70 indicate the general direction of airflow. Air enters the blower at the inlet 30 and is accelerated by the rotating impeller. The rotation imparted by the impeller generally directs the airflow in a tangential direction T. The volute then constrains the airflow to spiral the volute. The airflow then exits the blower in a generally tangential direction T via the outlet 20.
In some blowers, such as axially developed volute blowers, the volute geometry directs the tangential spiraling airflow in a slight axial direction A prior to exiting the blower in a generally tangential direction T.
The performance of a blower is often described using fan curves, which show the flow rate of air versus outlet pressure of air. Many factors affect the fan curve including impeller diameter and the number and shape of the impeller blades. The design process is a complex balance between competing priorities such as desired pressure, flow rate, size, reliability, manufacturability and noise. While many combinations of size, shape and configuration of components may produce a flow of pressurized air, such a result may be far from optimal, or be impractical.
ResMed Axial Volute Design
Another form of known blower design is described in ResMed's International Patent Application PCT/AU99/00444, published as WO 99/64747, the contents of which are hereby expressly incorporated by reference. As described in this patent application, the volute geometry develops in a generally axial direction, however air exits this blower in a generally tangential direction.
Respironics Ventilator
Respironics International Patent Application PCT US98/19635, published as WO 99/13932, describes a medical ventilator which has a blower assembly that preferably includes three rotating impellers and two stationary stators. In this device, a conventional volute design is used such that air exits the blower assembly in a generally tangential direction.
A disadvantage of this blower design is it tends to suffer from blade pass tonal noise emission.
Respironics REMstar
Another known blower is found in the Respironics REMstar series of CPAP devices. In this device, air exits the blower in a generally tangential direction.
ResMed Blowers
U.S. Pat. No. 6,910,483 (Daly et al) assigned to ResMed Limited describes a double ended variable speed blower for Continuous Positive Airway Pressure (CPAP) ventilation of patients that includes two impellers in the gas flow path that cooperatively pressurize gas to desired pressure and flow characteristics. The contents of this patent are hereby expressly incorporated by reference. In this device, air exits the blower in a generally tangential direction.
PCT Application Nos. PCT/AU2006/001617, filed Oct. 27, 2006, and PCT/AU2006/001616, filed Oct. 27, 2006, describe multiple stage blowers. The contents of both of these PCT applications are hereby incorporated by reference.
As noted above, known CPAP and VPAP blowers use a more or less conventional volute design, namely one where the air leaves the volute tangentially. These designs have the disadvantage that the asymmetry of the volute leads to asymmetry of flow patterns in the volute and impeller. This problem is especially significant at flow rates away from the ideal “design” flow rates of the volute. CPAPs and VPAPs, unfortunately, are used for a substantial portion of their operational time under non-ideal flow conditions as a consequence of very high excursions in the flow demand. This means that the flow patterns within the volute, and consequently within the impeller, become highly asymmetrical, uneven, and even unstable. This in turn leads to pressure pulses and turbulence. As a consequence, acoustic blade pass tonal noise and turbulence noise are produced.